All references cited in this specification, and their references, are incorporated by reference herein where appropriate for teachings of additional or alternative details, features and/or technical background.
Disclosed in the embodiments herein is a document illuminator comprising a light-transmissive element having a compound curved concentrator profile, such as a compound parabolic concentrator profile.
Document illuminators are generally used in systems for scanning documents. The illumination source typically includes a tungsten lamp or fluorescent lamp and an opposing reflector as described in U.S. Pat. No. 6,236,470 which is incorporated in its entirety by reference herein. Although such illumination systems may be adequate for general use, they are spacious and costly to maintain.
An improved illumination source is provided in a related application, U.S. patent application Ser. No. 10/995,462 filed on 23 Nov. 2004, wherein a document illuminator comprising a small size light source, such as a light emitting diode (LED), in a light-transmissive element as shown in FIGS. 1a-2b is disclosed. As shown in FIG. 1a, the light-transmissive element 105, which performs the function of a light guide, comprises a part of the document illuminator 100 for scanning a document 10 on a platen 20. The light guide is configured to incorporate an LED 110 in a manner so as to provide a powerful and uniform illumination of the document in a compact size and form. FIGS. 2a and 2b show an enlarged view of the LED document illuminator. It will be noted from the cross-sectional view in FIG. 2a that the LED document illuminator 100 has substantially straight walls. As set forth in U.S. patent application Ser. No. 10/995,462, it has been found that the luminescence emanating from the document illuminator can be further strengthened and made more uniform by modifying the light guide to have nonlinearly shaped walls as schematically depicted in FIG. 1b. 
FIG. 1a illustrates the placement of an LED document illuminator 100 in relation to a platen 10, and shows a document handler 20 configured to feed document 30 to be scanned by the document illuminator. Light guide 105 comprises a light source region 110, which emits light that is directed out of the light guide as rays 120 to illuminate the document 10 on platen 20, as shown in FIG. 1a. An imaging system 43, comprising a lens, causes an image of the portion of the document immediately surrounding optical centerline 40 to be projected onto a light sensing device 45 comprising a linear array of photo-sensors (for example, a full width array sensor), a charge coupled device (not shown), or a photoreceptor (not shown). The light reflected from the document about optical centerline 40 is converted by the light sensing device 45 into electronic signals forming image data which electronically represent the document, and the data may be stored on a recording device such as a memory storage device in a computer.
FIGS. 2a and 2b of U.S. patent application Ser. No. 10/995,462 show a front view and a side view, respectively, of document illuminator 100 having substantially straight walls. The document illuminator is capable of illuminating page width documents ranging from, but not limited to, letter and legal size to A3, A4 sizes. An LED 130 is fitted inside a cavity 140 formed in the light-transmissive element or light guide 105. The cavity shown in FIGS. 2a and 2b is centrally located within the light-transmissive element. Light emanating from a light source 150 inside the LED is coupled into the light guide through the walls of the cavity. A chamfered aperture 160 is formed in the light guide to receive light either directly or reflectively, from the LED and redirect it to illuminate a document (not shown) adjacent the aperture. LED 130 may be held inside the cavity in a number of ways, including mounting the LED on a circuit board, which in turn forms a shoulder support 135 for the LED against the sidewall of the light guide 100, as shown in FIGS. 2a and 2b. The circuit board may be a component of an electronic system (not shown) for controlling the light source 150 of the LED 130. Cavity walls are polished to aid in the unimpeded transmission of light from the LED 130 to other parts of the light guide 100.
In general, light rays 151 (shown in solid arrows) emitted by the LED 130 will emanate radially in all directions from cavity 140, some refracting 153 and escaping into the surrounding environment, some others reflecting 155 (shown in dashed arrows) back into the guide, and bouncing back and forth before leaving the guide altogether. Ray 155 may go through, what is called, a retroreflection such that ray 155′ is reflected parallel to the original ray 151. Some rays will travel directly into the region of the aperture 160 and project 167 onto the surrounding area, including the document to be illuminated. It will be noted that a ray, such as 151 striking the chamfered surface of aperture 160 will refract into the surrounding environment medium in the direction 167 shown in FIG. 2a. 
The power and uniformity of light emanating from aperture 160 may be enhanced by guiding the light to reflect from the inside walls of the light-transmissive element 105 with reduced escaping into the surrounding environment before it is collected and then diffused out through the aperture for illuminating a document. In FIGS. 2a and 2b, the mostly internally reflected light rays are collected at an opaque diffuser 170. It is usually difficult to prevent all light rays from escaping into the environment. For example, light rays 151 traveling east and south-eastwardly in FIG. 2a have a loss component 153. To minimize such losses, it is possible to judiciously place opaque specular linings or light blockers 143 and 143′ on certain portions of the cavity wall as shown in FIG. 2a. That is, as described in the related application of Ser. No. 10/995,462, most of the light rays that would have otherwise escaped are conserved and reflected back from specular blockers 143 and 143′. Consequently, most light rays, such as 151 traveling south-westward in view of FIG. 2b from LED 140, go through total internal reflection, that is, without any loss to the surrounding environment, as depicted by dashed rays 155, and arrive at diffuser 170 with more power than otherwise, and emanate from aperture 160 with a more uniform profile than otherwise.
As is known, total internal reflection of a ray of light at a boundary between two dissimilar media occurs at angles of incidence θi (measured from the normal to the boundary) greater than a critical angle θcr at which the ray can be refracted at a refraction angle θr=90° to the normal, that is, parallel to the boundary surface. For any light ray in the light-transmissive element 110 having an incident angle greater than θcr, none of the light ray will escape from the light-transmissive element into the surrounding medium, thus yielding total reflection from the boundary back into the light guide, without any transmission of refracted light into the surrounding medium. This phenomenon which occurs at θcr is known as total internal reflection.
Disclosed herein is a light transmissive element, or light guide, having nonlinearly shaped walls associated with a light source so as to provide enhanced total internal reflection for light rays emanating from the light source as described below and shown in FIGS. 3a and 3b. 